Firing and press furnaces for dental restoration parts are used for subjecting the dental restorations that mostly consist of glass ceramics, to a heat treatment in order to achieve a condition of the dental restoration part that is ready for the intended use. Temperatures of up to 1200° C. are required for the heat treatment in firing and press furnaces.
The quality of the produced dental restoration part to a large extent depends on the temperature profile and the temperature distribution in the dental furnace during the firing and/or pressing operation of the dental ceramic part. If the temperature gradients during the firing- or pressing operation in the dental restoration part during the heat treatment get too large, the quality of the product produced drops. In some cases this may even lead to an erroneous result, which in turn means that the dental technician's work of several hours invested before is lost. Moreover, the existing temperature gradients in a dental furnace restrict the dental technician's possibilities and/or reduce the robustness of the process.
The respective selection and the adjustment of the optimal parameters, but also the insertion of the dental restoration part into the firing furnace or the press furnace, i.e. for example the kind of firing tray or the position of the dental objects thereon during the heat treatment process, or the object positions in a press muffle regularly are up to the choice of the dental technician and strongly depend on his experience. Basically, the temperature gradients in the dental firing spaces mentioned before, in particular with short and quick thermal treatment processes, result in many conflicting situations that are difficult to solve. This is even more the case if multi-unit bridges etc. or a plurality of small dental restorations together are subjected to a heat treatment at the same time.
In particular in press muffles it is predefined for the dental technician via a sprue guide for mounting the wax parts in a press muffle, how far to the edge and how far into the center the objects are sprued. This limitation reduces the possibilities and also the number of units that can simultaneously be pressed, or the process times must be extended in general and clearly.
Further it has been proposed according to DE 199 05 666 A1 and corresponding U.S. Pat. No. 6,252,202, which is hereby incorporated by reference, to provide a lower heating element in addition to a lateral heating element in order to avoid cold surfaces in the firing space and in order to provide uniform heating in the entire heating chamber. With the aid of quartz covers a further homogenization of the radiation of heat and simultaneously some protection is to be achieved. A lower heating element of this kind, however, requires serious additional effort since the firing furnaces are typically configured as hood or top hat furnaces in which the electrical resistance heating is provided in the hood thereof whereas the bottom of the firing furnace is free from electrical connections. Also for safety reasons it is not surprising that a furnace of this kind has not made its way because the provision of additional heating elements in the floor of the firing space also requires that an absolutely safe muffling or casing of the heating element at this position must be carried out which muffling in turn obstructs the heat transfer.
Typically, the firing space floor is strongly polluted by deposits that burn into the floor. Therefore, intensive detergents to ensure the cleaning of the floor also require the use of moisture or wetness such as water. With a furnace having a refractory or fire-proof ceramic plate it is acceptable to place it under water, this however not being the case if electrical connections are located thereunder.
In this respect, the existing course of events in dental laboratories must be changed, and an additional special provision for furnaces of this kind must be put into practice.
The reference mentioned above indeed suggests the use of an insulation plate for the floor of the firing space. This, however, is unfavorable for thermal reasons since the heat transfer from the lower heating element into the firing space is impaired by this.